Impact strength is a measurement of the ability of a material to resist high rate loading of force. The evaluation of impact strength is used for a number of purposes. Impact strength is used to preliminarily screen materials to determine if the materials have sufficient resistance to breakage when impacted with a force of the magnitude expected to be encountered in normal use of a product made from the material as well as under anticipated abnormal conditions such as when the product is accidently dropped.
Impact strength testing is also used for quality control evaluations to determine the variations, if any, from batch to batch of material and more importantly, to determine if a given batch of material has the specified properties required to make the end product. Impact strength monitoring is particularly important in the manufacture of plastic products. The plastic compositions used to mold the products are generally made of a number of distinctly different components some of which are highly subject to variations in properties as a result of minor changes in formulations and processing conditions. An example of the type of plastic compositions which must be closely monitored for impact strength are those used for pressing records, especially capacitive electronic discs. Record molding compositions typically include a theomoplastic resin such as a polyvinylchloride resin which itself can vary widely from batch to batch, fillers such as carbon black, and various additives such as lubricants, processing additives, antistatic agents and the like. The addition of fillers, and in particular, carbon black, to record molding compositions is known to have a significant detrimental effect on the impact strength of record molding compositions. The addition of relatively large amounts of carbon black is, however, essential in the manufacture of certain types of records, such as capacitive electronic discs, in order to obtain the electrical conductance required for capacitance electronic playback. Because of the close and adverse relationship between impact strength and carbon black loading quality control testing of impact strength is exceptionally important in the record pressing art. Impact strength testing is, of course, also an important quality control tool in the manufacture of other products in addition to records.
Impact strength testing is also widely used in experimental development of new compositions. The effect of changes in the chemical composition, processing conditions and other similar variables can be relatively simply determined by measuring the affect on impact strength.
Various methods had been suggested to measure impact strength. One of the most widely used tests is the Izod impact test which is described in ASTM Standard D-256. In the lzod impact test, a sample of a specified standard size and shape is prepared with a notch cut into the sample at a predetermined position. The sample is clamped in a stationary position and a weighted pendulum is released from a known height so as to contact the test sample with a known force to determine the impact strength of the material under evaluation.
Another commonly used test, generally referred to as the dropped weight test or the Gardner test, is described in ASTM D 3059. In this test, a test piece of a known weight is dropped from a known height onto a stationary test specimen of the material under evaluation. The calculated force developed by the test piece required to break the test specimen is the impact strength.
Tests of the above type have not been found to be fully correlatable with the impact strength properties of products made from the tested material. The variations in impact strength properties which are obtained on the test sample and the actual product are believed to be due in part to the differences in the stresses formed during molding of the product and also the conditions under which impact forces are typically applied to the actual product as compared to the standard size test sample. In the conventional tests, such as the Izod test or the Gardner test, the material to be evaluated is held in a stationary position and is struck with an accelerating, moving object, i.e., the pendulum or the falling test piece. However, under conditions normally encountered actual use of many products, it is the product which is normally the accelerating, moving element which makes contact with a stationary body. This occurs, for example, when a molded record is dropped wherein the record is the moving element and the floor, or the like, is the stationary body which is contacted by the record. A further possible cause of the differences in impact strength values obtained under the standarized test conditions and in actual use can be attributed to the difference in the shape of standard test pieces and that of the actual end product. The test samples are made in a specified shape, such as a rectangular bar for testing. The end product, however, usually has a substantially different shape from the standard test piece and these differences can substantially effect the results obtained on impact.
Suggestions have been made of test methods to determine impact strength properties which are more relevant to conditions of actual use by employing test methods designed to simulate the actual drop conditions encountered by the final product. This type of testing is, however, limited to objects which have a weight or a shape or a combination of both such that the force of gravity will result in a predictable, repeatable free fall path. A type of test apparatus which can be used for evaluating products such as loaded cartons is disclosed by Ford et al in U.S. Pat. No. 3,224,249 entitled "Adjustable Fixture For Drop Testing." Aerodynamically shaped articles can also be tested by free fall drop of the final product. Such a type or apparatus is disclosed in Bergs et al in U.S. Pat. No. 3,426,578 entitled "Impact Test Apparatus" discloses a test apparatus in which a specimen, such as an artillery shell having an aerodynamically stable shape, is evaluated to determine its impact resistance.
Free fall impact testing apparatus of the type heretofore suggested in the prior art cannot be utilized, however, to satisfactorily evaluate the impact strength properties of lightweight products which are inherently aerodynamically unstable. Lightweight aerodynamically unstable articles when dropped will tend to tumble during free fall and cause uncontrollable variations in the forces developed during the free fall and also uncontrolled variations in the point of impact from test to test. The results obtained under these test conditions are so erratic that the test results are of little in any value. In this regard, it has been found to be especially difficult to evaluate thin disc shaped articles having a diameter which is substantially greater than the thickness of the article, for example, a disc having a diameter of about 50 or more times than its thickness. A specific product of this type which has heretofore been essentially impossible to evaluate with any degree of reliability by free fall drop testing has been molded plastic records. A typical long playing record has an outer diameter of about 12 inches (30.48 cm), a thickness of about 1/16th of an inch (0.16 cm) and is made of a relatively lightweight filled plastic composition. The unfavorable ratio of the outer diameter to the thickness of a molded record tends to cause tumbling in drop testing the effect of which is further complicated by the presence of the center hole which is conventionally formed in records as the center hole appears to further increase the instability of records during free fall.
Ideally to evaluate the impact strength of a molded record the record should be allowed to vertically free fall so as to be impacted vertically at a point on its outer diameter edge. This type of impact would impart the greatest concentration of impact force through the body of the record which could be expected to be encountered in actual use, and thus, represents the best test condition for evaluating free fall breakage characteristics of the record. As indicated above, if a conventional record is allowed to free fall, it will tumble in an uncontrolled manner and almost never will land squarely on the outer diameter edge and the results obtained are erratic and unreliable with regard to the impact strength characteristics of the molded record.
What would be highly advantageous would be an apparatus for evaluating impact strength in which an aerodynamically unstable article, such as a disc or the like, could be subjected to controlled vertical free fall so as to impact consistently on the outer diameter edge thereof so as to obtain repeatable, reliable results with regard to impact strength.